A whole reflection light quantity signal of a sine wave obtained when an optical spot on an optical disk moves in a direction of a radius of the optical disk is required to be deformed into a signal having states of "H" and "L" varying on timing when the whole light quantity signal traverses a center of its amplitude. Such a whole reflection light quantity signal has a DC component and an AC component and its level varies on variation in light quantity and in a reflection ratio of the optical disk. In a prior art reference, the deformed signal is provided by comparing the whole reflection light quantity signal with a comparison signal obtained by averaging the whole reflection light quantity signal. However, since such a comparison signal is obtained by using a time constant circuit such as a low pass filter, steep variation in the level of the whole reflection light quantity signal disturbs the comparison signal from following its variation and this causes the deformed signal to be never stably obtained.